1. Field of the Invention
The present invention relates to an uninterruptible fluorescent lamp circuit available for emergency lighting, and particularly relates to a fluorescent lamp circuit comprised of a oscillatory boosting circuit and a current-limiting circuit for replacement of conventional starters and stabilizers. The present circuit effectively reduces the flash of fluorescent lamp and reduces the possibility of burning out of the fluorescent lamp due to excessive instantaneous current when activating the lamp.
2. Description of the Prior Art
All kinds of conventional fluorescent lamp circuits include stabilizers and starters. The main functions of stabilizers are:
1) Limiting the current passing through a lamp. The resistance of a fluorescent lamp is very high before discharge, i.e., before the lamp is lighted; then, the resistance abruptly drops after discharge, i.e., after the lamp is lighted. A very large current passes through the lamp after the same is discharged and it might very possibly burn out the lamp if there is not a stabilizer installed. Accordingly, one of the main functions of stabilizers is to avoid an over current or excessive current passing through a fluorescent lamp. For this reason, the stabilizer is also referred to as an impedance coil.
2) Helping the discharge of a fluorescent lamp. A stabilizer is a wound coil, so, it has inductance. In the instant that a starter breaks the circuit, this inductance induces a voltage which is connected in series with a power-supply voltage. The inductance voltage is applied to the two ends of the fluorescent lamp, causing the lamp's voltage at its ends to significantly increase and help the discharge of the lamp.
When a thermal tube in a lamp discharges, the filament should be preheated. Once the discharge begins, the filament keeps discharging without needing subsequent heat. A starter may immediately close the filament circuit in the instant the voltage applies to the fluorescent lamp, and, automatically open the circuit when the filament temperature is increased.
When classified by the types of starters, one of the most commonly used starters is the glow starter. Such starter is made of a small tube in which a connection point of an U-shaped doublelayer metal plate is sealed. When the lamp voltage is increased, a glow discharge occurs inside the lamp. The double-layer metal plate is heated and therefore contacts fixed electrodes and connects the two electrodes of the lamp, causing the same to heat. When the two electrodes are connected, the glow disappears, the double-layer metal plate cools and separates from the fixed electrodes. In the instant the double-layer metal plate separates from the electrodes, the stabilizer generates an induced voltage which together with the power-supply voltage causes the thermal tube of the fluorescent lamp to discharge and thereby, lights the lamp. When the lamp is lighted, the above-mentioned starter immediately stops the glow discharge and no heat is generated between its two electrodes. Thus, a starter is used to shortly conduct current between the two ends of a lamp and to heat the thermal tube of the lamp when the lamp is switched on; then, when the lamp discharges, the starter automatically opens the circuit to increase the lamp's voltage at ends for helping to light the lamp.
Fluorescent lamps with the above-mentioned stabilizers and starters, however, have the following disadvantages in their actual applications:
1. The power source maintains the same frequency and the lamp has an obvious flicker which has an adverse effect on a user's vision.
2. Poor starters can easily blacken the lamp.
3. In the instant the stabilizer effects, the lamp must bear an instantaneous high current pulse which has adverse effect on the usable life of the lamps.
4. The lamp is easily blackened or frequently has intermittent flash when the power-supply voltage is too low or when the lamp is switched on and off too many times within a short period of time. Reversely, when the power-supply voltage is too high, it might cause the lamp to be overheated which will directly affect the usable life of the lamp.
5. Poorly installed stabilizer shall emit noise.
It can be seen from the above that conventional fluorescent lamp circuit design is apparently not an ideal one.